While developing Power Pack 6 against Review Board 6, a bug was found

where the JavaScript and CSS bundles for the PDF and Office Document review

UIs failed to load on the review UI pages. This only happened on the dev

server when using an installed package of Power Pack 6. This bug didn't

affect production servers. It also didn't affect Review Board 7 dev servers,

which can run with a development install of the Power Pack package.
To fix this, we now use Djblets' ext_js_bundle and ext_css_bundle

template tags instead of Django Pipeline's javascript and stylesheet

tags. The Djblets tags essentially do the same thing as Pipeline's and

call on the Pipeline code in their implementation. The difference is that

the Djblets tags use the ExtensionStaticMediaNodeMixin, which allows them

to render static media for both development setups and installed packages

of extensions on dev servers.
This fix relies on a Djblets fix (/r/14390) which will land in 4.0.1.

However, this is backwards compatible with older versions of Djblets in

the sense that nothing will break if Review Board runs with an older version

of Djblets that doesn't include the fix there. The Djblets fix enables

ext_css_bundle and ext_js_bundle to do the right when passing full

bundle IDs from a review UI's js_bundle_names and css_bundle_names

attributes. When a server is running with a version of Djblets that

doesn't include the fix, the Djblets tags will return nothing and then

this will fall back on using Pipeline's javascript and stylesheet

tags, which will do the right thing on production servers. One annoying

thing with the backwards compatibility is that when the Djblets tags

return nothing, they'll also output a log warning complaning that the

bundles names don't exist, since the bundle names are the full IDs

instead of relative names which older versions of Djblets can't handle

properly.

Ran unit tests.
Ran JS unit tests.

Tested this patch on the following:

- RB6 production and dev servers that contain the Djblets fix.

- RB6 production and dev servers that don't contain the Djblets fix.

- RB7 production and dev servers that contain the Djblets fix.

- RB7 production and dev servers that don't contain the Djblets fix.